WO2023052973A1

WO2023052973A1 - Carbon dioxide sequestration method and reactor

Info

Publication number: WO2023052973A1
Application number: PCT/IB2022/059154
Authority: WO
Inventors: Anthony Owen FILMER; Luke Mark KEENEY; Roland Gunter Berndt
Original assignee: Anglo American Technical & Sustainability Services Ltd; PIENAAR, Danie
Priority date: 2021-09-28
Filing date: 2022-09-27
Publication date: 2023-04-06

Abstract

This invention relates to a method and reactor for the sequestration of carbon dioxide (CO2). The method includes the steps of providing a reactor 10 comprising a porous heap of particulate material capable of CO2 sequestration stacked on an impermeable pad 12 and enclosed within a coating 16 which is substantially impermeable to gas flow. CO2 gas 18 is supplied into the enclosed heap, to maintain a CO2 partial pressure greater than 0.1 within the heap; and providing the heap with water 20 whereby free water content in the heap is maintained at less than 30% by weight of the heap.

Description

Carbon Dioxide Sequestration Method and Reactor

BACKGROUND OF THE INVENTION

Sequestration of carbon dioxide (CO2) provides an opportunity to reduce CO2 emissions in the transition to a zero net carbon economy, and to better manage the ongoing emissions from the many carbon sources that are unlikely to be substituted.

Examples of CO2 generators which are unlikely to be substituted range from large point sources such as cement and steel production, through to smaller localized sources such as breweries or waste incinerators.

To date, large-scale storage of CO2 has been confined to the collection and use of CO2 as an in-situ displacement fluid for enhanced oil and gas recovery. (IPCC Special Report on Carbon Dioxide Capture and Storage, Cambridge University Press, 2005)

The CO2 from various point sources is collected, upgraded, and pumped into the porous rock formations from which the oil is being recovered. The intent is to ultimately seal the CO2 in-situ and store it for an extended period, and thus avoid it polluting the atmosphere. The CO2 point sources are remote from the oilfields suited to such in-situ storage, and hence the off-gas from the preceding processes is chemically purified, typically using amine pressure swing reactors, to near 100%. The gas is then compressed and pumped through pipelines up to several hundred km in length, to the point of injection, (ieaghg pipeline report 2013)

The cost of such purification and transportation is substantial, and hence any losses of the purified CO2 to the atmosphere are detrimental to both the environment and the economic benefits of such disposal.

A quite different method of permanent CO2 removal can be achieved by reaction with naturally occurring rocks to form carbonates. This is termed carbon sequestration. The potential of ultramafic rocks to sequester CO2 is well known, with extensive theoretical studies performed over many years. (Yadav . Environmental Science and Pollution Research (2021 ) 28:12202- 12231 )

Sequestration relies upon the chemical reaction of CO2 with the rock to generate an insoluble carbonate, and as such it represents permanent storage of CO2. It is this very slow sequestration occurring over an extended surface area of rock that ultimately removes much of the CO2 in the atmosphere, thus maintaining the long-term equilibrium CO2 content that recent human activity is disrupting. Hence the efforts to accelerate this natural reaction to offset the increasing CO2 levels in the atmosphere.

Many laboratory studies, usually in agitated reactors, have shown that the rate of the sequestration reaction is proportional to the surface area of reacting rock and the CO2 concentration. The sequestration reaction is exothermic; and it has a high activation energy and hence is strongly dependent on temperature. Different mafic and ultramafic rocks, both naturally occurring and produced by other industrial processes such as smelter slags and fly ash, react at very different rates. The benefits of such permanent sequestration over underground storage are self-evident, with no potential for leakage or rupturing of the storage location, and the closer proximity of suitable rocks close to the locations of point sources of CO2 emissions. However, the potential to pump CO2 rich gasses into the earth (in-situ sequestration) is limited by porosity of the rocks, particularly as the molar volume of the solid expands as the sequestration reaction proceeds. Again, any losses of the purified CO2 source piped in from a remote CO2 source, subtract from the value of sequestration by such methods.

Many possible pathways for ex-situ sequestration, where the rock is excavated and then used for sequestration have been proposed. (Kelemen et.al An Overview of the Status and Challenges of CO2 Storage in Minerals and Geological Formations, Front. Clim., 15 November 2019)

The key to success of such ex-situ sequestration is to accelerate the sequestration reaction such that it can occur in a residence timesuited to the reactor in which the CO2 containing gas, the rock and water are mixed. This residence time is relevant for both the gas and the solids in the reaction vessel.

The proposed pathways for accelerating the sequestration rate, range from creating the conditions for fast reactions by increasing temperature (around 150^eC) such as in a kiln; and also pressurizing with CO2 in an autoclave; through to chemically converting the rock into highly reactive species like magnesium hydroxide, then exposing this species to CO2 containing gas. In the latter, finely ground rock is leached to dissolve the magnesium or calcium rich solutions, which are subsequently exposed to CO2 in the presence of an alkali to generate magnesium hydroxide, which reacts rapidly with the CO2 at acceptable rates under ambient conditions.

In yet another approach to sequestration, natural atmospheric sequestration has been considered using existing mine residues, relying on a very high surface area of rock. An example is the very fine flotation residues from nickel recovery that are currently stored as tailings (Wilson et. al. Offsetting of CO2 emissions by air capture in mine tailings at the Mount Keith Nickel Mine, Western Australia: Rates, controls and prospects for carbon neutral mining, International Journal of Greenhouse Gas Control. Volume 25, June 2014, Pages 121 -140).

On measurement, the tailings on the immediate surface of the tailings dam showed clear evidence of sequestration, albeit at a very slow rate, but the CO2 could not penetrate more than a few centimeters into the saturated residue. Hence the achievable rate of sequestration in stored tailings was not consistent with meaningful levels of CO2 absorption by the solids.

The waste rock heaps from a historic asbestos mine has also been found to naturally sequester CO2. April 2016, International Journal of Greenhouse Gas Control 47:110-121 , Karl Lechat, et al.

The rate of sequestration is such that the CO2 content of the air within a shallow heap of the waste rock was much reduced from the usual atmospheric 400ppm of CO2. This also demonstrated that the rate of sequestration was at least partially constrained by the access of sufficient air into the waste rock. Whilst the airflow through the rock heaps can be increased in part by good heap design, the rate of sequestration from natural airflows through large heaps will be limited. The time for the available surface area of solids to be converted to carbonate is excessive.

One proposed solution to this constraint of the necessary CO2 flux into a purpose designed heap, would be to preconcentrate the CO2 and inject this gas it into the heap. There are various proposals on methods of upgrading CO2 from the atmosphere. The use of amine pressure swing mentioned previously is one such method. But these preconcentration methods have a very high cost of generating pure CO2, especially when concentrating to high CO2 concentrations from the very low levels of CO2 in the natural atmosphere. And even where a point source of CO2 exists, the costs of upgrading and transporting the high-grade CO2 to a waste rock heap are significant. The available surface area and sequestration reactions are too low for efficient extraction of CO2 to occur as the CO2 containing gas passes through the rock heap, even with a large heap of waste rock. Instead, much of the high-cost concentrated CO2 is simply vented through the heap and back into atmosphere.

In a variant of sequestration using the high surface area of rock heaps, Walder has proposed combining sequestration with heap leaching. (US 9,194,021 B2). The high magnesium pregnant liquor is generated using a heap leach irrigated from the surface with an acid. This leaching takes place in the heap, with a countercurrent flow of CO2 containing gas directed upwards from the base of the heap, with any unreacted CO2 escaping from the surface of the heap. The countercurrent flow allows CO2 to acidify the down-coming leaching liquid, thus further leaching cations from the rock. The liquor containing the leached cations and carbonates is collected from the base and then further processed to recover metals of value and precipitate the CO2. Presumably, this subsequent sequestration could be achieved by the technique such as that previous described in which magnesium hydroxide is precipitated with caustic soda, then reacted in a separate vessel with CO2.

In summary, despite all the studies over a few decades, and many different proposed pathways for ex-situ sequestration, no practical method of achieving the sequestration has been found.

It is an object of the invention to provide an improved method for the sequestration of CO2 which allows for the capture of most or all of the CO2 contained in the gas.

SUMMARY OF THE INVENTION This invention relates to a method for the sequestration of carbon dioxide (CO2), including the steps of: providing a porous heap of particulate material capable of CO2 sequestration stacked on an impermeable pad and enclosed within a coating which is substantially impermeable to gas flow; supplying CO2 gas into the enclosed heap, preferably to maintain a CO2 partial pressure greater than 0.1 , preferably 0.2 atmospheres, and preferably greater than 0.5 atmospheres, and up to 2 atmospheres within the heap; and providing the heap with water whereby free water content in the heap is maintained at less than 30% by weight and preferably between 10- 20% by weight of the heap.

By “particulate material capable of CO2 sequestration” means material that reacts with CO2 and water to produce an insoluble carbonate.

Typically, the p80 of the particulate material forming the heap is in the range 1 -10mm, and preferably around 2-3 mm; and the p10 is greater than 0.1 mm, and preferably around 0.2mm.

The particulate material may be generated from crushed rock or other material suited to sequestration of CO2; and may consist of a smaller grain size of less than 0.2mm, that has been agglomerated together or on to a larger substrate.

The particulate material may be supplemented with unsaturated fines with a typical size of less than 150 microns, such that the particulate material provides a path for CO2 to migrate through the heap to the surrounding fines. The concentration of CO2 fed into the heap has preferably been preconcentrated to greater than 90% CO2, and preferably greater than 95% CO2, prior to addition to the heap.

Preferably, an average CO2 content measured as a proportion of dry gasses contained in the heap, is maintained at >0.5 and preferably >0.8, and even more preferably >0.9.

High contents of CO2 can be maintained by extraction from the enclosed heap and purification of this gas, prior to reinjection into the heap, or by venting to atmosphere.

The heap may be formed from a mix of fine powders of p80 less than 0.15 mm interspersed between particulate material, providing the fine powders are maintained in an unsaturated state, and hence enabling gas flow through both the particulate matter and the fine powders in the heap.

Typically, the exothermic sequestration reaction is utilised to increase the temperature in the enclosed heap to greater than 30°C, and preferably to greater than 50°C and even more preferably around 60°C.

The heap preferably includes a covering layer of insulating material over the upper and side components of the seal, to retain the exothermic heat of reaction within the heap.

Supplementary heat may be provided to the heap by external heating of the CO2 or water or steam that is being injected into the heap.

A multiplicity of enclosed heaps may be used with fluid flow between the heaps to utilise the CO2 more efficiently prior to venting to atmosphere, and for transfer of heat between heaps. A multiplicity of heaps may also be used to balance flows of CO2 with the variable sequestration rates of the enclosed rock that occur, depending on the degree of sequestration in a particular heap.

A multiplicity of heaps may also be used when the gas source to be sequestered has a lower input proportion of CO2, such as to use the heaps in series to strip out most of the CO2 prior to venting the gas to atmosphere

The particulate material may be a residue arising from prior processing of rock ore for the purposes of recovering other values, such as nickel or diamonds, or asbestos. The particulate material may also be byproducts or wastes from other industrial processes such as fly ash or slags.

Sequestration may be utilised to precondition the ore for the subsequent processing for recovery of values from the sequestration products.

Sequestration products may be utilised for recovery of a saleable product such as MgCO3.

Additives may be used to accelerate the reaction, and recovered from the sequestered heap for reuse in subsequent heaps

The heap may be operated under a total pressure of between 0.9 and 2 bar, to enable a high CO2 partial pressure, typically greater than 0.5 atmospheres, even as operating temperatures increase.

This invention also relates to a reactor for the sequestration of carbon dioxide (CO2), comprising: a porous heap of particulate material capable of CO2 sequestration stacked on an impermeable pad and enclosed within a coating which is impermeable to gas flow; supply means for supplying CO2 gas, typically at the base of the heap; and irrigating means for supplying water, typically at the top of the heap.

The particulate material may be a residue arising from prior processing of rock ore for the purposes of recovering other values such as nickel, diamonds or asbestos.

The particulate material may be a residue from other industrial processes such as a fly ash, or a smelter slag.

The particulate material may also be a mineral containing ore which has not yet been processed, in which the carbonation is used to selectively alter the host rock, making it easier to process.

The coating may comprise welded geotextile, or shotcrete, or bitumen, or any other material that can create an impermeable barrier.

Preferably, the heap includes a covering layer of insulating material over the upper and side components of the seal, to further reinforce the seal and retain exothermic heat of reaction within the heap.

The insulating layer may comprise the particulate matter, hence forming the start of another heap to be enclosed, or any other material that can restrict convectional and conductive heat loss. Connections may be installed in the coating to enable the flows of gas and liquids into or out of the heap as may be required.

The height of the particulate material in the heap may be greater than 5m and less than 100m, and typically between 10 and 30m.

The heap may also be dynamic in nature, utilising a fixed base, walls and roof to form a space into which the particulate material is added, the structure is the sealed and the rock sequestered, and on completion the sequestered particulate material is removed and replaced with fresh particulate material to continue the sequestration process.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is schematic illustration of a reactor for the sequestration of CO2, according to an embodiment of the invention; and

Figures 2A-F are graphs showing particle carbonation kinetics of olivine rocks with different particle size and with varying temperature.

Figure 3 is a graph showing the reaction kinetics of various sources of fine particulate matter containing 30% water, in an atmosphere containing CO2.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to Figure 1 , the present invention relates to a practical 3- dimensional enclosed reactor indicated by the numeral 10, and a method of making and operating the reactor 10, to deliver the requisite conditions for sequestration of CO2, over the weeks, months or years required for the sequestration material to achieve a high level of carbonation, whilst also capturing most of the CO2 as it passes through the reactor. The rate of sequestration is dependent on both surface area of the sequestration material and the temperature. To illustrate the potential for natural sequestration in a suitably designed 3-dimensional structure, laboratory data has been normalized to approximate the sequestration rate for wetted particles of olivine rocks, at various sizes expected, and at different temperatures, when exposed to a pure CO2 atmosphere. The normalization shown in Figures 2A to 2F guides the design criteria for crush sizes, heap operating temperature, and residence times.

The rate of sequestration has been further confirmed through reacting various finely divides materials in sealed vessels in which the amount of CO2 absorbed was measured by the pressure drop, with the CO2 content within the vessel being ‘topped up’ intermittently. The effect of material type, temperature, CO2 partial pressure, and water content can be ascertained from the data in Figure 3. The trends in the data are broadly in line with the expectations based on literature data, that underpin Figure 2

With reference back to Figure 1 , rock suitable for sequestration of CO2 is crushed and/or agglomerated to create competent particles with sufficient pore space and size between the particles to enable bulk gas permeation. The fines can either attached to coarser particles by agglomeration, or separated and pelletised to form particulates, or removed by classification. The resultant competent particles have a p80 diameter of between 1 and 30mm, and preferably between 2-3mm, and with a p10 of greater than 0.05mm and preferably greater than 0.1 mm.

Particles of crushed and/or agglomerated particulates are stacked on an impermeable pad 12 to form a porous heap 14.

The heap 14 is enclosed within a coating 16 which is impermeable to gas flow and is applied around the external surfaces of the heap 14.

Carbon dioxide (CO2) gas is supplied into the enclosed heap 14, typically at the base 20, of the heap 14, at around 1 atmosphere pressure, such as to maintain a CO2 partial pressure greater than 0.2 atmospheres, and preferably greater than 0.5 atmospheres, to cause CO2 18 to flow though the heap 14.

The particles of crushed and/or agglomerated rock are wetted prior to stacking and additional water 22 is supplied as required by irrigation into the enclosed heap 14, typically at the top 24 of the heap 14 and water 20 flows downwardly through the heap 14, at a rate sufficient to maintain free water content at less than 30% by weight and preferably between 10-20% by weight of the heap.

A bleed point 26 is provided at the top of the heap 14 to allow the enclosed gas to bleed from the top of the heap, and a bleed point 28 is provided at the base of the heap 14, to allow for water egress (as required).

CO2 from the bleed 26 may optionally be purified 30 and recycled to the base 20 of the heap 14, or pumped to a separate heap for sequestering most of the remaining CO2 content.

The key elements of the invention are the enclosed and porous heap in which sequestration material with a high surface area is contained; together with a high partial pressure of CO2 which as it is consumed creates an area of low pressure that draws in additional CO2; and in which the sequestration surface is wetted but not saturated such as to promote gas permeability through the heap; with the wetted surface providing for both dissolution of CO2 and dissolution of the sequestration material; and in which the exothermic sequestration raises the temperature within the sealed heap to accelerate and sustain the reaction.

The sequestration material to be enclosed is crushed and where necessary agglomerated to an upper size where it provides for geotechnical stability of the heap. The average sizing provides for effective permeability of gas through the heap, and in so doing ensures high partial pressures of CO2 at reactive surfaces and distributes heat through the heap. Through the use of a fixed heap, the geotechnical stability can also be provided by the walls of the fixed structure.

If excessive fines exist at the crush size required for the required surface area for the sequestration to progress both rapidly and extensively, the fines can be agglomerated either together or on a coarser substrate. Agglomeration can also be utilised if the material has already been comminuted for other purposes, such as previous flotation of metals, or from another process such as fly ash.

The fines can also be interspersed around the coarser material such that the coarser material provides the bulk transfer of fluids through the heap, allowing diffusion to occur into the interspersed and unsaturated fines.

The heap of crushed sequestration material is sealed and maintained at around 1 atmosphere pressure, such that meaningful quantities of CO2 and water vapour do not escape from the enclosed heap.

This seal enables sufficient gaseous residence time in the heap for a high conversion of CO2 to the sequestered state. The seal also contains the exothermic heat of sequestration within the enclosure, thus further accelerating the rate of sequestration. The seal also prevents the entry of air into the heap, thus maintaining the internal atmosphere in the heap at high levels of CO2.

Whilst the seal prevents meaningful quantities of air ingress to the heap, some level of air contamination of the injected CO2 will exist. This may originate from the purification of the CO2 at the point source, or from air in the first fill that is not flushed out, or leaks in the seal. This air contamination can be managed by flushing to atmosphere, or purification of the gas circulating within the enclosed heap, or by passing the off gas through another heap of particulate matter. The heap can also be operated at pressures above 1 bar, enabling the use of even more elevated temperatures whilst maintaining a satisfactory CO2 partial pressure.

Optionally, a bleed stream of the gas from within the heap can be processed to reconcentrate the CO2 within this gas stream prior to its re-injection into the heap, thus maintaining a high partial pressure of CO2.

The sealed heap is connected to the source of CO2. The reduced pressure that arises as the sequestration occurs within the sealed heap, enables a natural distribution of CO2 as pressure is equalized within the heap.

The availability of water at the sequestration material surface is required to promote the sequestration. This film of water is originally created through wetting the sequestration material prior to stacking, then equilibrated over time by the flow of water vapour within the heap. As required, additional water, optionally containing other chemicals to accelerate the reaction, is irrigated from the top and migrates slowly down through the heap at rates such that the rock surfaces remain wetted but unsaturated. Excessive quantities of water, for example >30% by weight free water, will impede the flow of CO2 through the heap.

The temperature of the enclosed crushed sequestration material increases as sequestration occurs due to the exothermic nature of the sequestration reaction, thus further accelerating the sequestration rate.

Due to the presence of water within the heap, the temperature increase is limited by the vaporization of water, and hence thermal runaway cannot occur. Optionally, the reaction can be supplemented by preheating either the CO2 gas or the water irrigating the heap.

The sequestration reaction can be utilised in conjunction with recovery of other values from rock sequestration material. Depending on the rock type, the sequestration may be used to complement residue storage from activities such as diamond or nickel mining or iron and nickel smelting; or used prior to the value recovery to break apart the rock matrix to enable easier recovery of the values from the sequestration residue.

It is also possible to reclaim the sequestered rock for subsequent processing to refine and utilise the resultant magnesium carbonate component.

High levels of sequestration of ultramafic rocks, slags, and other reactive materials such as fly-ash, can be achieved through use of an enclosed heap into which a high-grade source of CO2 is added.

The first key element of the invention is a suitably enclosed heap. This enclosure enables sufficient residence time for the CO2 to react with the crushed sequestration material, and hence limit the losses of CO2 passing through the heap to the surrounding atmosphere.

The heap can be constructed to retain a seal between the heap and the surrounding atmosphere. As examples the heap can be located on a sealed base, constructed to the preferred height and shape, irrigation added to the top of the heap, and then enclosed. This external enclosure can be made from welded geotextile, or shotcrete, or bitumen, or any other material that can create an impermeable barrier to gas flow. Connections through this barrier can be installed to enable the flows of gas and liquids into or out of the heap as may be required, whilst maintaining the overall heap seal.

Ideally the heap will be a large walled or free standing, three-dimensional structure, shaped to minimise the area to be sealed relative to the volume of rock that is to be sequestered. Whilst there are no constraints on the maximum length and width of the heap, the heap height affects the structural integrity and the surface area to volume ratio affects heat loss during the reactions. The preferred height of the heap is greater than 5m and less than 100m, and preferably between 10 and 30m. Over-stacking on previously sequestered heaps can be considered. In one embodiment, to enable expansion of the heap within the enclosed volume as sequestration proceeds, fines can be agglomerated onto the coarser crushed rock prior to stacking. Such agglomeration enables a very high surface area of rock and a large void space, thus avoiding excessive loss of macro-permeability of the heap.

The second key element of the invention is to construct a heap of crushed rock that retains sufficient porosity and structural integrity through the duration of the sequestration reaction, to enable CO2 gas to access the residual unreacted and wetted particulate surface.

Within the natural porous structure of a heap, the main constraint to gas transfer within the heap is where water closes the pore structure of fines. Hence most of the fines fraction, that may be placed in the heap adjacent to the particulate, must be in an unsaturated form.

The free moisture content of the heap is controlled be greater than 5% to provide water for the sequestration reaction, but less than 30% at which stage significant proportions of the heap become saturated thus restricting gas flow. Furthermore, at the upper end of the moisture range, the structural integrity of the heap becomes compromised. Ideally the free moisture content will be maintained between 10-20% by weight.

In the event that sequestration products consume water, additional water can be added to the heap.

To achieve these heap permeability objectives, the upper size of the particles in the heap should be greater than a p80 of 1 mm to ensure adequate gas permeability through the bulk dimensions of the heap, particularly in the later stages of sequestration when the increase in molar volume of the sequestered product will close much of the pore structure between particles.

Where the intention is to utilise a significant proportion of the chemical capability of rock, the p80 particle size must be less than 10mm, as the rate of sequestration is slowed significantly due to the larger particle size. The preferred upper particle size to balance the needs of sequestration reaction rate and gas permeability and heap porosity is a p80 of between 2-5mm.

Optionally, the heap can be constructed with a substrate structure that is either not reactive or only partially reactive. This substrate structure provides structural integrity to the heap, preventing collapse of the heap as the sequestration reaction progresses. The p80 of such a substrate is greater than 10mm, but less than 30mm.

The average size of the individual mineral grains in the heap must be sufficiently small to provide adequate surface area to enhance sequestration rates. The p50 of the mineral grains to be sequestered should ideally be less than 1 mm, and preferably less than 0.5mm and even more preferably less than 0.05mm. However, to maintain sufficient porosity the mineral grains can be agglomerated together into porous agglomerated particles, with a p50 greater than 0.2mm, and preferably greater than 1 mm, and preferably around 2-3mm.

The third key element of the invention is to maintain a high partial pressure of CO2 throughout the duration of the sequestration reaction. Depending on the location of the heap relative to the point source of CO2, this can be achieved in different ways.

The preferred embodiment for achieving a partial pressure of CO2 is to inject CO2 at concentrations >95% and preferably greater than 99%, into the enclosed heap. The heap can be flushed of the pre-existing air, by injecting CO2 at the base, and venting air from the top. This injection of high-grade CO2, together with effective sealing of the heap, enables substantive sequestration to occur before impurities such as nitrogen and oxygen build up to the point of requiring further venting. Alternatively, the stacked and sealed heap can be partially evacuated to reduce the amount of air present, prior to filling with CO2 to commence the sequestration.

Where there are impurities in the CO2 gas supplied to the heap, or inadvertent leaks in the seal that allow air ingress, these impurities will accumulate as the CO2 is removed by sequestration, and become increasingly concentrated within the enclosed heap. In such a case, a bleed stream of gas from the enclosed heap is required to maintain a high partial pressure of CO2.

One embodiment of the invention utilises the bleed stream from the heap and utilises CO2 purification techniques such as amine swing reactor, to remove the impurities. The CO2 is then recycled to the heap.

After accounting for the vapour pressure of water, the partial pressure of CO2 in the heap is maintained at greater than 0.2 atmospheres, and preferably greater than 0.5 atmospheres, and even more preferably around 0.8 atmospheres. The proportion of CO2 in the enclosed gas in the heap is preferably greater than 50%, and even more preferably greater than 80% of the dry gas composition.

In a second embodiment, the bleed stream from the heap can be passed through one or more sequential enclosed heaps such as to preheat these heaps, flush air from the enclosure, and commence sequestration such that the CO2 in the bleed stream from the final heap is mostly consumed before release of the contained gas to the atmosphere.

In a third embodiment, typically utilised where the point source and ultramafic rock are in close proximity, the gas injected to the heap will not be purified, but rather be injected at the CO2 partial pressure existing at the point source, typically between 15-30% CO2. In this case sequential enclosed heaps are required to achieve CO2 extractions of preferably greater than 50% prior to venting to atmosphere. In this embodiment, the gas is directed into the first heap where sequestration occurs, progressively reducing the CO2 content of the off-gas as it is directed to the second and potentially subsequent heaps required to scrub the CO2 to an acceptable level for venting to atmosphere. As the desired sequestration capacity of the solids is reached in the first heap, this heap is taken offline, with off-gas assigned to what was the second heap in series, and an extra heap of fresh rock is introduced at the end of the series of heaps. This configuration of heaps enables an effective use of the heat of reaction to heat the fresh crushed rock, and the countercurrent flow of rock and gas through the reactors retains acceptable CO2 removal even as much of the sequestration capacity of the first heap is consumed.

The fourth key element of the invention is to achieve an elevated temperature within the heap. This thermal energy accelerates the sequestration rate, as is illustrated in Figure 2, where every extra 20°C in reaction temperature reduces the residence time required for sequestration by a factor of around 10. The enthalpy of the sequestration of olivine rock by direct aqueous carbonation is around 90 kJ/mole (Yadav et. al Carbon storage by mineral carbonation and industrial applications of CO2, Materials Science for Energy Technologies, Volume 3, 2020, Pages 494-500)

This heat of formation corresponds to just under 0.5 kJ/g of reactants, which at a specific heat of around 1 J/g, illustrates that the exothermic reaction is more than adequate to heat an enclosed heap. Excess heat will be absorbed by the vaporization of water, which will limit the maximum reactor temperature.

By enclosing and insulating the heap, the only forms of heat loss are the conductive transfer through the enclosing surface from the internal reactor to the surrounding atmosphere, and the heat transferred by the addition or removal of reactants. Because gas flows within the heap are limited to the addition of CO2 at the rate that it is sequestered, and there is a static layer of rock immediately inside the enclosing surface, which will be cooler than the bulk, the heat losses are small. In a large heap this depth of cooler layer will be negligible.

Heat will be generated uniformly through the heap, as all the solids throughout the heap will react with the surrounding CO2 atmosphere at broadly similar rates. Hence heat redistribution will not be necessary, over and above natural convective processes that will occur within the heap

The operating temperature within the heap will increase as the reaction proceeds, and then cool through heat losses to the surrounding environment as the reactions approach completion. The preferred maximum temperature will be between 25°C and 90^eC, and preferably around 50^eC to 60°C.

In a separate embodiment, if required for initiation of the temperature rise, or by a relatively slow sequestration rate of the rock, the reaction can be initiated or sustained by adding either preheated CO2 or preheated water or steam to the heap.

In the case where temperatures approach or exceed 100°C, the heap will be operated under a slight positive pressure to maintain sufficient partial pressure of CO2 in the reaction zone.

In yet another embodiment, the heap can be covered with an insulation layer such as sand, or a selected insulating material, or the commencement of particulate material for a new enclosed heap, to further reduce the convective heat losses on the external side of the enclosing seal.

This insulation layer may be incorporated with the seal on a freestanding heap, or specially designed as part of the fixed base, walls and roof in a dynamic heap.

And in a third separate embodiment relating to temperature, hot gas from one sequestration heap can be transferred to a fresh heap to raise the temperature of the reactants, hence initiate or sustain the required reaction temperature in the fresh heap.

Once the sequestration reaction has proceeded to the point where further reaction is very slow, the heap can be left to slowly absorb the remaining CO2 present within the enclosed space, and then opened to the atmosphere to achieve further sequestration over geological time.

In one embodiment, the sequestered residue can be recovered from the enclosed heap at the completion of sequestration and used for a variety of purposes. These include use on an as produced basis, or in a further beneficiated form where the magnesium carbonate is separated from the residual silica rich rock. Examples of uses of the sequestered rock or beneficiated product are as a soil additive, a neutralising agent, or a fire retardant.

In yet another embodiment, the sequestration reaction can be performed in conjunction with recovery of other values. This value recovery can occur prior to sequestration, in which the ore to be sequestered is a residue from the comminution and beneficiation process used to recover the values. An example would be the recovery of nickel by flotation or heap leaching, prior to utilising the residue for sequestration.

Alternatively, the sequestration process can be used as a method of liberating the values by altering the encompassing gangue mineral structure, thus aiding subsequent comminution and beneficiation. An example would be the recovery of diamonds by sequestering the surrounding gangue to soften it substantively, thus enabling recovery of the diamonds without concerns about fracture of the large stones during comminution. A second example would be the liberation of nickel from the silicate matrix present in ultramafic nickel ores. A third example might be the pre-conditioning of ores which are difficult to crush and grind. In summary, the current invention creates the equivalent of a controllable highly porous in-situ CO2 sequestration site, in which sequestration occurs over a short duration due to the high surface area and controllable water content and elevated temperature, and where the sequestration products can be validated and ultimately recovered and further processed.

Example

Figure 3 shows the reactivity of CO2 in the presence of various forms of ultramafic rock containing 30% by weight water. The rocks have been ground to less 75 micron and are placed in the base of a sealed reactor at 70^eC with an overpressure of CO2.

Table 1 - Test Conditions

Table 2 - Model Form CO@ Absorption= C1 *Davs^AC2

The Canadian ultramafic and Brazilian slag were tested in an atmosphere of 17% CO2, whilst the Finnish ultramafic test utilised 100% CO2.

The reaction rates are consistent with direct carbonation occurring in a sealed heap of particulate material, and progressing up to a CO2 content of around 0.1 tonnes of CO2 per tonne of rock, over a period of one to two years (i.e., around 30 to 40% of their theoretical maximum sequestration capacity).

Considering the passive nature of sequestration occurring in enclosed heaps, and the logarithmic nature of the reaction kinetics, further additions of CO2 should enable ongoing sequestration to even higher levels of CO2 per tonne of rock.

Claims

24 CLAIMS

1 . A method for the sequestration of carbon dioxide (CO2), including the steps of: providing a porous heap of particulate material capable of CO2 sequestration stacked on an impermeable pad and enclosed within a coating which is substantially impermeable to gas flow; supplying CO2 gas into the enclosed heap; and providing the heap with water whereby free water content in the heap is maintained at less than 30% by weight.

2. The method claimed in claim 1 , wherein the free water content in the heap is maintained at 10-20% by weight.

3. The method claimed in claim 1 , wherein the CO2 gas is supplied into the enclosed heap to maintain a CO2 partial pressure greater than 0.1 atmospheres within the heap.

4. The method claimed in claim 3, wherein the CO2 gas is supplied into the enclosed heap to maintain a CO2 partial pressure greater than 0.2 atmospheres within the heap.

5. The method claimed in claim 4, wherein the CO2 gas is supplied into the enclosed heap to maintain a CO2 partial pressure greater than 0.5 atmospheres within the heap.

6. The method claimed in claim 1 , wherein the p80 of the particulate material forming the heap is in the range 1 -10mm, and the p10 is greater than 0.1 mm.

7. The method claimed in claim 6, wherein the p80 of the particulate material forming the heap is in the range 2-3 mm; and the p10 is around 0.2mm

8. The method claimed in claim 1 , wherein the particulate material is generated from crushed rock, or other material less than 0.2mm that is agglomerated prior to addition to the heap.

9. The method claimed in claim 1 in which the particulate material is supplemented with unsaturated fines with a size less than 150 micron, such that the particulate material provides a path for CO2 to migrate through the heap to the surrounding fines.

10. The method claimed in claim 1 , wherein the concentration of CO2 gas fed into the heap is than 90% CO2.

1 1. The method claimed in claim 10, wherein the concentration of CO2 gas fed is greater than 95% CO2.

12. The method claimed in claim 1 , wherein an average CO2 content measured as a proportion of dry gasses contained in the heap, is maintained at >0.5.

13. The method claimed in claim 12, wherein an average CO2 content measured as a proportion of dry gasses contained in the heap, is maintained >0.8.

14. The method claimed in claim 13, wherein an average CO2 content measured as a proportion of dry gasses contained in the heap, is maintained at >0.9.

15. The method claimed in claim 12, wherein the content of CO2 in the heap is maintained by extraction of gas from the enclosed heap and purification of the extracted gas, prior to reinjection into the heap, or by venting to a second heap or to atmosphere.

16. The method claimed in claim 1 , wherein the heap is formed from a mix of in fine powders of p80 less than 0.1 mm interspersed between particulate material, providing the fine powders are maintained in an unsaturated state, and hence enabling gas flow through both the particulate matter and the fine powders in the heap.

17. The method claimed in claim 1 , wherein an exothermic sequestration reaction is utilised to increase the temperature in the enclosed heap to greater than 30°C.

18. The method claimed in claim 17, wherein an exothermic sequestration reaction is utilised to increase the temperature in the enclosed heap to greater than 50°C.

19. The method claimed in claim 18, wherein an exothermic sequestration reaction is utilised to increase the temperature in the enclosed heap to around 60°C.

20. The method claimed in claim 17, wherein the heap is covered with layer of insulating material over the upper and side components of the seal, to retain the exothermic heat of reaction within the heap.

21 . The method claimed in claim 17, wherein supplementary heat is provided to the heap by external heating of the CO2 or water or steam being injected into the heap.

22. The method claimed in claim 15, wherein a multiplicity of heaps are used, with fluid flow controlled between the heaps to utilise the CO2 contained in vent gas and transfer heat between heaps.

23. The method claimed in any one of claims 15, wherein a multiplicity of heaps is used to balance flows of CO2 with the variable sequestration rates of the enclosed rock that occur, depending on the degree of sequestration in a particular heap. 27

24. The method claimed in 15, wherein a multiplicity of heaps is used when the gas source to be sequestered has a lower input proportion of CO2, such as to use the heaps in series to strip out most of the CO2 prior to venting the gas to atmosphere

25. The method claimed in claim 1 , wherein the particulate material is a residue arising from prior processing of rock ore for the purposes of recovering other values.

26. The method claimed in claim 25, wherein the particulate material is a residue arising from prior processing of rock ore for the purposes of recovering nickel or diamonds or asbestos.

27. The method claimed in claim 25, wherein the particulate material is byproducts or wastes from industrial processes.

28. The method claimed in claim 27, wherein the particulate material is fly ash or slag.

29. The method claimed in claim 1 , wherein the particulate material is ore and sequestration is utilised to precondition the ore for subsequent processing for recovery of values from the sequestration products.

30. The method claimed in claim 1 , wherein the sequestration products are utilised for recovery of a saleable product including MgCO3.

31 . The method claimed in claim 1 , wherein additives are used to accelerate the reaction, and recovered from the sequestered heap for reuse in subsequent heaps

32. A reactor for the sequestration of carbon dioxide (CO2), comprising: 28 a porous heap of particulate material capable of CO2 sequestration stacked on an impermeable pad and enclosed within a coating which is substantially impermeable to gas flow; supply means for supplying CO2 gas; and irrigating means for supplying water.

33. The reactor claimed in claim 32, wherein the CO2 gas is supplied at a base of the heap, and water is supplied at a top of the heap.

34. The reactor claimed in claim 32, wherein the p80 of the particulate material forming the heap is in the range 1 -10mm; and the p10 is greater than 0.1 mm.

35. The reactor claimed in claim 34, wherein the p80 of the particulate material forming the heap is in the range 2-3 mm; and the p10 is around 0.2mm

36. The reactor claimed in claim 32, wherein the heap includes a layer of insulating material over the upper and side components of the seal, to retain exothermic heat of reaction within the heap.

37. The reactor claimed in claim 32, wherein the particulate material is a residue arising from prior processing of rock ore for the purposes of recovering other values.

38. The reactor claimed in claim 37, wherein the particulate material is a residue arising from prior processing of rock ore for the purposes of recovering nickel, diamonds or asbestos.

39. The reactor claimed in claim 32, wherein the coating comprises welded geotextile, or shotcrete, or bitumen. 29

40. The reactor claimed in claim 32, wherein the insulating layer includes insulating particulate matter that can restrict convectional and conductive heat loss.

41 . The reactor claimed in claim 40, wherein the insulating particulate matter is crushed rock.

42. The reactor claimed in claim 32, wherein connections are installed in the coating to enable the flows of gas and liquids into or out of the heap as may be required.

43. The reactor claimed in claim 32, wherein the height of the particulate matter in the heap is greater than 5m and less than 100m.

44 The reactor claimed in claim 43, wherein the height of the particulate matter in the heap is between 10 and 30m.

45. The reactor claimed in any claim 32, wherein the structure of the external containment is a fixed structure with access for introducing and removing particulate material, in which the contained particulate material is stacked, then sealed and sequestered, then removed from the structure.